This application is based on application No. 2001-380902 filed in Japan, the content of which is hereby incorporated by reference
1. Field of the Invention
The present invention relates to a light source device and an optical pickup, in particular, to a light source device that emits a divergent beam having a light intensity distribution in approximately a circular form and to an optical pickup (in particular, a high density optical recording and reproduction pickup) using the same.
2. Description of the Prior Art
A semiconductor laser, such as of an LD (laser diode), is used as a light source in a conventional optical pickup. Since a divergent beam emitted from a semiconductor laser has a cross section in an elliptical form (that is to say, the light intensity distribution is in an elliptical form), it is necessary to convert an elliptical beam from a semiconductor laser to a circular beam in an optical recording and reproduction device wherein a high light utilization efficiency is required. In practice, an anamorphic prism is used as a beam shaping element for reshaping the form of the beam cross section from an elliptical form to a circular form. However, it is necessary to utilize an anamorphic prism in a parallel light flux and adjustment is difficult, and miniaturization is limited. On the other hand, a finite system-type beam shaping element using an anamorphic lens or a diffraction optical element is known as a beam shaping element for reshaping the cross sectional form of a divergent beam from an elliptical form to a circular form. Such a beam shaping element can be placed in the vicinity of a semiconductor laser and is effective for integration and for miniaturization.
However, a high precision for positioning, in the optical axis direction, the semiconductor laser relative to a beam shaping element is required in the beam shaping element formed of the above described anamorphic lens, and the like. In particular, the absolute amount of wave front aberration that is tolerable becomes more restricted due to the shorter wavelength in the beam shaping element of a high density optical recording and reproduction device using a next-generation blue semiconductor laser. Accordingly, a higher positioning precision than ever is required in the optical axis direction. It is difficult to restrict the occurrence of astigmatic difference due to change in the environmental temperature, even in the case that such a high positioning precision can be achieved. The light source device proposed in U.S. Pat. No. 5,467,335 is cited as a light source device having the object of being stable in regard to change in environment. However, such a light source device is difficult to manufacture because the distance between the beam shaping element and the semiconductor laser is extremely small (approximately several tens of xcexcm).
A purpose of the present invention is to provide an improved light source device and an optical pickup.
Another purpose of the present invention is to provide a light source device that can emit a divergent beam in approximately a circular form while restricting the occurrence of astigmatic difference due to change in the environmental temperature and that is easy to manufacture, as well as to provide a compact optical pickup using the same.
The above described purposes are achieved by providing a light source device having the following configuration:
a semiconductor laser for emitting an elliptical divergent beam having a light intensity distribution in an elliptical form; and
a beam shaping element for converting an elliptical divergent beam from the above described semiconductor laser into an approximately circular divergent beam having a light intensity distribution in an approximately circular form;
wherein the beam shaping element has a first surface formed of a cylindrical surface and a second surface formed of an anamorphic surface, in order from the above described semiconductor laser side, and satisfies the following conditional expressions (1) and (2) when the direction parallel to the minor axis of the ellipse of the elliptical divergent beam is the X direction and the direction parallel to the major axis of the ellipse of the elliptic divergent beam is the Y direction:
xcex8X less than xcex8Xxe2x80x2 less than nxcex8Xxe2x80x83xe2x80x83(1)
xcex8Y greater than xcex8Yxe2x80x2 greater than (1/n)xcex8Yxe2x80x83xe2x80x83(2)
where
xcex8X is the angle of spread in the X direction of the elliptical divergent beam from the semiconductor laser;
xcex8Y is the angle of spread in the Y direction of the elliptical divergent beam from the semiconductor laser;
xcex8Xxe2x80x2 is the angle of spread in the X direction of the approximately circular divergent beam from the beam shaping element;
xcex8Yxe2x80x2 is the angle of spread in the Y direction of the approximately circular divergent beam from the beam shaping element; and
n is the index of refraction of the beam having the design wavelength of the beam shaping element.
In addition, the above described purposes are achieved by providing a light source device having the following configuration:
a semiconductor laser for emitting an elliptical divergent beam having a light intensity distribution in an elliptical form; and
a beam shaping element for converting an elliptical divergent beam from the above described semiconductor laser into an approximatly circular divergent beam having a light intensity distribution in an approximately circular form;
wherein the beam shaping element has a first surface formed of a cylinder surface and a second surface formed of an anamorphic surface in order from the above described semiconductor laser side and is characterized by satisfying the following conditional expressions (3), (4) and (5):
n greater than 1.6xe2x80x83xe2x80x83(3)
1.1n less than M less than 1.5nxe2x80x83xe2x80x83(4)
s/txe2x89xa0(nxe2x88x92M/n)/(Mxe2x88x921)xe2x80x83xe2x80x83(5)
where
n is the index of refraction of the beam having the design wavelength of the beam shaping element;
M is the shaping magnification of the beam shaping element;
s is the distance between the light emission point of the semiconductor laser and the first surface of the beam shaping element; and
t is the thickness of the core of the beam shaping element.